Pretty much every day someone asks me if there's a tool for solving a specific API problem. While Runscope solves many API problems (particularly debugging, testing and monitoring) for app developers, there are many things we don't do. My goal with this post is to list out all the API tools that I know of and frequently recommend to people. Some even compete with us, but the more the merrier.

I've left out code libraries and frameworks since that would make the post too long.

Cloud-based Debugging Proxies

API Tools - Track, transform and analyze the traffic between your app and the APIs you use. (Free)

Desktop Debugging Proxies

Charles - Charles is an HTTP proxy / HTTP monitor / Reverse Proxy that enables a developer to view all of the HTTP and SSL / HTTPS traffic between their machine and the Internet. Supports OS X, Linux and Windows. (Paid)

Fiddler - The free web debugging proxy for any browser, system or platform. (Free)